An objective in the area of electrical machines in general is to make them more effective, such that they produce more torque with less weight and lower cost. Two electrical machine types have been in focus to full-fill this goal. These machines are the Permanent Magnet Transverse Flux Machine (PMTFM), and the switched reluctance machine (SRM). The PMTFM is well known for its high torque per weight ratio, but it is quite expensive and very difficult to manufacture because many parts are required. The SRM is one of the cheapest machines to manufacture and also has a reasonably good torque per weight density. Although the SRM has some good characteristics, it is not widely used mainly because of the high investment required to develop machines for an application and the high volume production and sales needed to lower the per unit cost of this development.
Electrical machines have traditionally been constructed by making a two dimensional cross-section in the X-Y plane and then extruding it in the axial dimension (z-axis) with a given number of non-oriented steel sheets. Such a two dimensional cross-section is shown in FIG. 1. The machine shown in FIG. 1 is a three-phase SRM with six stator poles 11 and four rotor poles 13. This machine has the disadvantage of long flux-paths in the stator yoke 15 from stator pole to stator pole and though the rotor yoke 12. The bobbin/needle wound coils 14 around the stator poles also present a disadvantage by extending past the steel stack thus making the machine longer. In addition, said coils are exposed and unprotected. With high magnetic saturation, which often is the case for an SRM, mutual couplings between the phases increases which makes exact control and design of the machine very difficult.
An alternative to the classical SRM in FIG. 1 is described in U.S. Pat. No. 5,543,674. This machine is made with U/C-cores and ring coils and as a disadvantage requires 3 stacks to make a three-phase machine. The machine is characterised by the transverse flux principle where the flux travels from pole to pole in the axial direction and not a radial direction (in the X-Y plane) as the classical machine in FIG. 1. Electrical machines utilising the transverse flux principle are primarily known for a high torque per volume density where the torque ideally increases linearly with the number of poles. Due to the fact that 3 stacks are needed, this machine requires many parts and is thus very complicated to manufacture.
U.S. Pat. No. 5,015,903 describes a switched reluctance machine with C/U-cores in the X-Y plane. The machine can be considered as a kind of alternative to the classical SRM where C/U cores are used. This machine has short flux-paths where only on a minor part of the stator yoke is magnetized during its operation. The machine uses two coils per C/U which require many parts. Also, the copper outside the C/U is not participating actively in torque production. Thus, the machine has the same disadvantages with many stacks and parts as the PMTFM, and the machine is therefore difficult to manufacture.
In U.S. Pat. No. 4,748,362 an SRM with bifurcated teeth is presented. This machine can be considered as a classical SRM with C/U cores at the end of the poles. This machine does not have a short short-flux path, but it is known for the similar properties as the transverse flux machines where the torque doubles due to the bifurcated teeth (dual teeth/poles). The machine has disadvantages of long-flux paths, small space for the coils and coils that are difficult to install.
It is an object of the present invention is to design an electrical machine, which solves the above-mentioned problem.